When you have dandruff, there are an unusually large number of skin cells flaking off your scalp. Dandruff can be itchy and irritating, and a little embarrassing.
Most people have a least a little bit of dandruff, since the shedding of skin cells from the scalp is perfectly normal. Dandruff only becomes a problem if your head is constantly itchy and your shoulders are always covered in white flakes. While we know the word dandruff dates from the 16th century, much of its origin is uncertain. The last part, -ruff probably comes from East Anglian dialect, huff or hurf, "scab."